The present invention is a method and apparatus for developing sheets of light sensitive photothermographic or heat developable film. Light sensitive photothermographic film typically includes a thin polymer or paper base coated with an emulsion of dry silver or other heat sensitive material. Once the film has been subjected to photostimulation by optical means, such as laser light, it is developed through the application of heat.
Heat development of light sensitive heat developable sheet material has been disclosed in many applications ranging from photocopying apparatus to image recording/printing systems. The uniform transfer of thermal energy to the heat developable material is critical in producing a high quality printed results. The transfer of thermal energy to the film material should be conducted in a manner that will not cause introduction of artifacts. These artifacts may be physical artifacts, such as surface scratches, shrinkage, curl, and wrinkle, or developmental artifacts, such as non-uniform density and streaks. Numerous attempts to overcome the above mentioned artifacts have resulted in limited success.
The U.S. Pat. No. 4,242,566 describes a heat-pressure fusing apparatus that purports to exhibit high thermal efficiency. This fusing apparatus comprises at least one pair of first and second oppositely driven pressure fixing feed rollers, each of the rollers having an outer layer of thermal insulating material. First and second idler rollers are also included. A first flexible endless belt is disposed about the second idler roller and each of the first pressure feed rollers. A second flexible endless belt is disposed about the second idler roller and each of the second pressure feed rollers. At least one of the belts has an outer surface formed of a thermal conductive material. An area of contact exists between the first and second pressure feed rollers and allows the heat developable light sensitive sheet material to pass between two belts while under pressure. When an unfused (undeveloped) sheet of material is passed through the area of contact between two belts, the unfused sheet is subjected to sufficient heat pressure to fuse the development of the sheet of material. This apparatus, although useful for photocopying applications, will subject the sensitive material to excessive pressure. Excessive pressure can result in the formation of physical image artifacts, such as surface scratches and wrinkles, especially if the material is of polyester film construction.
In U.S. Pat. No. 3,739,143, a heat developer is described for developing light sensitive sheet material without imparting pressure to the sensitive coating while the sheet material is being heated. This developer includes a rotating drum cylinder and an electrically heated metal plate where it is partially covering the cylinder and spaced therefrom to define a space for the sheet material corresponding to the thickness of sheet material. The sheet material is guided through an opening to be wrapped around the rotating cylinder while heat is being applied by the metal plate partially covering the rotating cylinder. While this developer may satisfactorily develop paper-based heat-developable image, this developer is not well suited to develop polyester film base material having imprecise control of film heating and pressure application. In addition, the curled path can introduce curling artifacts when the polyester film material is used.
U.S. Pat. Nos. 3,629,549 and 4,518,845 both disclose developers having thermally insulating drums concentrically mounted within a heating member. Sheets of light sensitive material such as coated paper or coated polyester film are developed by being engaged by the drum and driven around the heating member. While the developers of this type may be suited well for paper coated light sensitive material, they tend to develop various artifacts in a polyester film with coated emulsion, such as scratches and nonuniform density development when the film sticks to the drum surface.
The development device disclosed in U.S. Pat. No. 3,709,472 uses a heated drum to develop strips of film. However, this device is not suitable for developing single sheets of film having soft coated emulsion layers.
U.S. Pat. No. 3,648,019 discloses another developer with a pair of heaters on opposite sides of a low thermal mass locating device, such as a screen assembly. Although portable, this developer is relatively slow and poorly suited for commercial applications.
Other photothermographic film developers include a heated drum which is electrostatically charged to hold the film thereon during development. Since the side of the film bearing the emulsion is not in contact with the drum or other developer components, it is not subject to sticking or scratching as in some of the developers discussed above. Unfortunately, the electrostatic system used to hold the film on the drum during development is relatively complicated and poorly suited for developers configured to develop larger sized sheets of film.
The U.S. Pat. No. 5,352,863 discloses a photothermographic film processor purported to be capable of quickly and uniformly developing large sheets of photothermographic film. This developer consists of an oven having a film entrance and exit; a generally flat and horizontally oriented bed of film support material mounted for movement within the oven along a film transport path between the film entrance and exit; and, a drive mechanism for driving the bed of material to transport the film through the oven along the path. The film support material, which is in the form of the padded rollers, is noted to have a sufficiently low thermal capacity to enable visible pattern-free development of the film as the film is transported through the oven. Unfortunately, this apparatus is relatively large and has not fully addressed the need to manage the thermal expansion and contraction of the imaging material to prevent, for example, wrinkling, nor the need to minimize the effect of convective currents during the thermal development of the imaging material.
In general, and as it is discussed in the background sections of the patents referenced above, the density of the developed image is dependent upon the precise and uniform transfer of heat to the film emulsion. Nonuniform heating artifact can produce an unevenly developed image density. Uneven physical contact between the film and any supporting structures during development can produce visible marks and patterns on the film surface.
It is evident that a continuing need exists for improved photothermographic film developers. In particular, there is a need for a developer capable of quickly and uniformly developing large sheets of polyester, emulsion- coated film without introducing physical and developmental artifacts that are described above.